Transcription-dependent and -independent signaling of RSK2 in cancer metastasis

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Metastasis is a complex biological process, which involves multiple signaling pathways. Therefore, a single agent is often insufficient and combination therapy is vitally important in the successful treatment of metastatic cancer. We reported that p90 ribosomal S6 kinase 2 (RSK2) promotes metastasis by activating multiple downstream substrates. In addition, we recently identified that glutamate dehydrogenase 1 (GDH1), a critical enzyme in the glutaminolysis pathway, provides anti-anoikis signals by activating CamKK2 and its downstream AMPK to regulate energy production, protecting cells from anoikis, and promoting metastasis in lung cancer. The effect of GDH1 is evident in LKB1-deficient cancer, where AMPK activation predominantly depends on CamKK2. This suggests that RSK2 and GDH1 are promising anti-metastasis targets. Indeed, targeting RSK2 or GDH1 by fmk (identified by our collaborator Jack Taunton at UCSF) or R162 (identified by our group) attenuated metastasis. We recently found that genetic or pharmacological inhibition of these two distinct signaling axes results in synergistic attenuation of cell invasion, migration, and anoikis resistance in LKB1-deficient lung cancer. Biological and metabolic studies revealed that the combined therapy further attenuates CREB activation. Intriguingly, GDH1 knockdown resulted in decreased activity of CamK4, another upstream kinase of CREB, suggesting that combined targeting of RSK2 and GDH1 may further attenuate CREB activity by inhibiting RSK2 and GDH1-CamKK2-CamK4 signaling involved CREB phosphorylation. In addition, combined targeting of RSK2 and GDH1 synergistically induced anoikis in LKB1 wild-type (wt) expressing cells and RSK2 directly phosphorylated LKB1, leading to AMPK activation. These data suggest that RSK2 may signal through LKB1 and AMPK to contribute to anoikis resistance in LKB1 wt cells. Thus, we hypothesize that RSK2 and GDH1 coordinately regulates AMPK and CREB in transcription- dependent and -independent manners to provide pro-metastatic potential in cancer cells. Thus, targeting both cellular and metabolic signaling by combination of RSK2 and GDH1 inhibitors is a promising anti-metastasis therapeutic strategy. We will use lung cancer with LKB1 or LKB1 null as a research platform. Three specific aims are proposed: (1) To demonstrate whether RSK2 and GDH1 coordinately provides anti-anoikis protection to cancer cells by activating AMPK through LKB1 and CamKK2, respectively; (2) To determine whether RSK2 and GDH1 together mediates migratory and pro-invasive signals by activating CREB and CREB transcription targets; (3) To validate whether RSK2 and GDH1 signaling correlates with metastatic potential in patient tumors and evaluate the therapeutic efficacy of targeting RSK2 and GDH1 in combination in treatment of metastatic cancers.